Pyrokinesis is one of the PSI Powers you retain from Psychonauts 1 in where you can hold the power to build up a sphere of influence. Once you let the power go, it'll ignite anything within the sphere, whether it be friend or foe (though most friends won't be affected by it). You can try to get this early on or you can wait until later in the game, where the amount of enemies attacking you will be greater or when you have upgraded Pyrokinesis to have a larger sphere. Either way, the condition is to just ignite three at once with your ability.